This invention relates to overcurrent circuit protecting apparatus such as circuit breakers, and more particularly to circuit breakers which are current limiting, i.e. they limit peak let-through current of short circuit fault currents to a value which should not damage the load.
It is known to use positive temperature coefficient (PTC) resistive elements in circuit breakers for limiting fault current flowing through the circuit breaker. Current limiting polymer (CLP) materials exhibit PTC qualities. When connected in circuit with the circuit breaker contacts, the CLP material responds to heat caused by fault currents flowing in the breaker to substantially increase the electrical resistance of the CLP element. The resistance can rise approximately two orders of magnitude to sufficiently reduce current flow through the CLP element. The CLP material alone may be unable to absorb all of the energy of fault currents and therefore it is also known to place a shunt resistor in parallel with the CLP element. The change in resistance of the CLP element causes the current to shift from the CLP element to the parallel shunt resistor, which absorbs part of the energy and helps the circuit breaker interrupt the current flow with lower let-through current and energy.